1. Field of the Invention
The present invention relates to a shift register for shifting an optical signal, and more particularly to a shift register having optically bistable elements. This kind of shift register is used in the field of optical electronics.
2. Description of the Related Art
In general, a device, in which a plurality of elements respectively take a bistable state and the state of an element is successively transferred to another adjacent element, is referred to as a shift register. A flip-flop circuit and a charge coupled device (CCD) have been known as elements which take an electrically bistable state.
Several kinds of shift registers having a plurality of optically bistable elements have been suggested. See, for example, Collection of Lecture Theses (The Spring National Meeting, 1990), The Institute of Electronics, Information and Communication Engineers, pp. 4-23; Technical Research Report OQE89-141, The Institute of Electronics, Information and Communication Engineers; and Extended Abstracts, p. 786 (The Autumn Meeting, 1990), The Japan Society of Applied Physics.
FIG. 8 shows a conventional shift register having a plurality of optically bistable elements. This shift register includes a p-GaAs substrate and a plurality of light-emitting thyristors provided on the p-GaAs substrate. Each thyristor is formed of a first p-GaAs layer (anode layer), a first n-GaAs layer, a second p-GaAs layer (gate layer), and a second n-GaAs layer (cathode layer), which are layered in this order. Each second p-GaAs layer is provided with a gate electrode 800. A voltage of -5 V is supplied to each gate electrode 800 through a resistance network by a dc power source V.sub.GA. Each second n-GaAs layer is provided with a cathode electrode. Three-phase clock voltages .phi.1, .phi.2, and .phi.3 are applied to each cathode electrode. The p-GaAs substrate is grounded, whereby a voltage of 0 V is applied to each first p-GaAs layer (anode layer).
When the clock voltage .phi.3 applied to the cathode electrode of the third light-emitting thyristor increases from a Low level to a High level at a certain time, the third light-emitting thyristor is turned ON and starts emitting light. Then, the electrical potentials of the gate electrode and the second p-GaAs layer (gate layer) become almost equal to the electrical potential (0 V) of the first p-GaAs layer (anode layer). Because of this, a current flows between the gate layer and the cathode layer of the third light-emitting thyristor through the resistance network. As a result, under the condition that the electrical potential (about 0 V) of the second p-GaAs layer (gate layer) of the third light-emitting thyristor is a peak value, electrical potentials stepwise distributed are applied to each gate electrode 800. Thus, smaller voltages are applied to thyristors which are positioned further away from the third light-emitting thyristor. For example, the voltage applied to the gate electrode 800 of the fourth light-emitting thyristor is higher than that applied to the gate electrode 800 of the first light-emitting thyristor.
When the clock voltage .phi.1 becomes High level at a subsequent time, a high voltage is applied to the respective cathode electrodes of the first and fourth light-emitting thyristors. At this time, only the fourth light-emitting thyristor, the gate electrode 800 of which has a relatively high electrical potential, is turned ON to start emitting light; while the first light-emitting thyristor, the gate electrode 800 of which has a relatively low electrical potential, is not turned ON.
After that, when the clock voltage .phi.1 is changed from a High level to a Low level, the third light-emitting thyristor is turned OFF to stop emitting light. Thus, the light-emitting state is shifted from the third light-emitting thyristor to the fourth light-emitting thyristor.
However, in the above-mentioned conventional shift register, it is required to provide each light-emitting thyristor with a gate electrode and to form a complicated resistance network on the substrate. Because of this, elements other than the light-emitting thyristors occupy a large area on the substrate, making it difficult to integrate a number of thyristors on the substrate.